The drill screw incorporates a drill section for drilling a lower hole. Recently, austenitic stainless steel has been proposed as a suitable material for drill screws. (Refer for instance to No. Jikkai-Sho-60-75461 official report, No. Jikkai-Sho-62-40320 official report or No. Tokkai-Shohei-2-142911 official report.)
This is because the high degree of corrosion resistance of austenitic stainless steel has aroused the attention of those interested in extending the life span of the screw by means which could not be achieved by conventional drill screws made of carbon steel. However, as austenitic stainless steel offers, generally speaking, less cold-workability than carbon steel, drill screws are usually made of a category of steel which has less hardening property in cold-working due to the addition of copper. (For instance, JIS SUS-XM7)
However, austenitic stainless steel has a Vickers hardness of only 300-400 degrees, even after heavy cold-working is applied. While this kind of drill screw can be used with aluminum, it does not offer sufficient hardness for sheet irons. For sheet iron, drill screws must be processed for increased hardness. However, as austenitic stainless steel is not hardened by tempering, it must be hardened by an alternative hardening process. The following measures are working processes with austenitic stainless steel:
a. Those in which the head and part of the screw portion of a drill screw are made of austenitic stainless steel, whereas part of the drill, as well as the screw portion consist of carbon steel tempered by carburizing. After the screwing part and drilling part having bewn welded together, the drill part and screw part are formed. (For example, refer to No. Tokkai-Shohei-2-142911 official report.)
b. Those in which a hardened layer is formed over all or part of the drill and screw sections of an austenitic stainless steel drill screw by carburizing and nitriding hardening process. (For example, refer to No. Jikkai-Sho-62-40320 official report.)
c. Those in which a hard film such as nitrided titanium, etc., is coated over all or part of the drill and screw portions of an austenitic stainless steel drill screw by application of the PVD (Physical Vapor Deposition) method or the like, or those in which a hardened film of nickel/phosphorus alloy is produced on the part as above by nonelectrolytic plating. (For example, refer to No. Jikkai-Sho-60-75461 official report as above.)
d. Those in which the screw top and screw portion are made of hard austenitic stainless steel and the drill part is made of processed and hardened carbon by carburizing and the two parts are jointed by pressing. (For example, refer to No. Tokkai-Shohei-2-142911 official report as above.)